Method and apparatus for mass separation



June 26, 1956 c. F. ROBINSON 2,752,501

METHOD AND APPARATUS FOR MASS SEPARATION Filed March 21, 1952 2Sheets-Sheet 1 SENS/N6 C/RCU/ T SAMPLE INLE T MAGNET POLE MAGNET POLE 70EVACUA TING SYSTEM SAMPLE INLET INV EN TOR.

CHARLES F. ROB/NS ON A T TORNE Y June 26, 1956 c. F. ROBINSON METHOD ANDAPPARATUS FOR MASS SEPARATION Filed March 21, 1952 2 Sheets-Sheet 2 FIG.4.

IN VEN TOR. C HA RI. 8 E ROBINSON A T TORNE Y United States PatentMETHOD AND APPARATUS FOR MASS SEPARATION Charles F. Robinson, Pasadena,Calif., assignor, by mesne assignments, to Consolidated ElectrodynamicsCorporation, Pasadena, Calif., a corporation of California ApplicationMarch 21, 1952, Serial No. 277,773

I 9 Claims. (Cl. 250-413) This invention relates to a method of massseparation and to a mass spectrometer and to a mass discriminating 'ionsource embodying such method. The ion source may be used by itself toaccomplish mass separation or may be used in conjunction with associatedresolving systems.

Mass separation in general, as utilized in the practice of massspectrometry, involves ionization of a sample under investigation,subjecting the ions to the effects of electrical or magnetic fields orboth, to effectuate spatial or temporal separation thereof as a functionof specific mass and selectively sensing the relative abundance of oneor more given masses. Ions exhibit characteristic movement in electricaland magnetic fields as a function of ion mass and of the nature of thefield. In the conventional 180 mass spectrometer resolving system ionsare propelled through a transverse magnetic field and follow curvedpaths therein, the radii of which are determined by the strength of themagnetic field and the specific mass of the ions. In this method of massseparation ions of a given mass are focused through a so-calledresolving slit for collection either by altering the strength of themagnetic field or by varying the velocity at which the heterogeneousbeam of ions is introduced into the magnetic field.

In the so-called crossed field method of mass separation ions aresubjected to mutually perpendicular electrical and magnetic fields underwhich stimulus the ions travel in cycloidal paths, again the pitch ofwhich is a function of specific mass and field strengths. In all of theforegoing, selective collection of ions of a given mass is based uponseparation of the ions in space under the influence of the applied fieldor fields.

I have now developed a method of mass separation wherein the directionof travel of ions, as they emerge from such a crossed field, is made useof to effectuate the desired selective mass separation. The invention inone aspect involves the method of mass separation which comprisesionizing a sample to be analyzed, orienting the -11;-

ions in a common plane, developing an electrical field perpendicular tothis plane and a magnetic field normal to the electrical field wherebythe ions are set in motion with cycloidal trajectories, the pitches ofwhich vary as a function of ion mass, and separating the ions byselectively collecting the ions traveling in a given direction at themoment of issuing from the influence of the electrical and magneticfields. The term plane is used to designate planar, laminar orpencil-shaped beams.

In this method the ions are permitted to move in cyin which massdiscrimination is inherent, which ion source may be used as a highlysensitive, comparatively low Patented June 26, 1956 resolution massspectrometer, or may be associated with an auxiliary resolving system toresult in a high resolution system. In one aspect, the invention isdirected to a mass discriminating ion source comprising means forestablishing a uniform electric field across a space, means forestablishing a magnetic field across the space normal to the electricfield, means for ionizing a sample to be analyzed in a region lyingnormal to the electric field whereby ions are propelled from this regionunder the influence of the electrical and magnetic fields in cycloidaltrajectories and means forming an outlet aperture adjacent the boundaryof the space and spaced from the region of ion origin whereby ions issuethrough said aperture with a direction of travel dependent upon the massthereof.

In its simplest embodiment the mass discriminating source describedabove may be provided wtih a collector electrode and a resolvingelectrode intermediate the collector electrode and the referred toboundary aperture. In this way only ions traversing the boundaryaperture with a direction of travel such as to carry them through theresolving aperture will reach the collector electrode. In thealternative, any one of the many present forms of resolving systems maybe associated with the ion source so that the ions issuing from theboundary aperture are subjected to additional ion discriminating stimuliwhereby high resolution is accomplished.

The ion source as described is highly sensitive, but at the same time ismore sensitive to intial energies than presently conventional massspectrometers of the same size and magnetic field strength magnitude. Asa consequence, resolution in the ion source itself may not be as high asdesired in the higher mass ranges because of initial energy spread inthese ranges. In such a situation an auxiliary resolving system, asmentioned, is indicated. In lower mass ranges, as for example mass 4,where electric fields may readily be made large compared to initialenergies, the ion source may be used as an integral unit to eifectuatedesired mass separation and is extremely sensitive in this use.

The invention will be more clearly understood by reference to thefollowing detailed description thereof taken in conjunction with theaccompanying drawing in which:

Fig. 1 is a diagram in sectional elevation of an ion source inaccordance with the invention illustrating application of the source asa mass spectrometer;

Fig. 2 is a transverse section taken on the line 2-2 of Fig. 1;

Fig. 3 is a diagram illustrating the operation of the apparatus of Fig.1;

Fig. 4 is a diagram of a mass spectrometer in accordance with theinvention wherein the mass discriminating ion source is used inconjunction with an auxiliary resolving system, in this case a full-waveradio frequency gap system; and

Fig. 5 is a diagram of a modified form of the mass spectrometer of Fig.4.

The apparatus of Figs. 1 and 2 includes a plurality of parallel spacedelectrodes 12, 13, 14, 15, 16, 17, 18, 19, 20 and 21 disposed in anenclosing envelope 24. The envelope is provided with an exhaust line 25connected to an evacuating system (not shown) and to a sample inlet line26 providing means for introducing a sample to be analyzed into the ionsource. A feature of the instrument is its suitability for use withsolid samples whereby the sample inlet 26 is representative of means forintroducing such a solid sample. The problem of ionizing a solid sampleis discussed in considerable detail in my co-pending application SerialNo. 277,772, filed March 21, 1952.

The several electrodes 13 through 21 are apertured to permit ionmovement in the field and are at one end provided with batfies toprevent exit of the ions from the field, except between electrodes 15,16. The opening between these electrodes, as shown at the right-hand endin Fig. 1, forms in effect an outlet aperture adjacent the boundary ofthe electrical field.

A transverse magnetic field is developed across the space by means ofmagnet poles 28, 29 so that ions formed in the electrode system aresubjected to transverse electrical and magnetic fields, whereby theymove in the field .in cycloidal trajectories.

Ions are formed in the source by means of a laminar electron beam 32directed across the source by a conventional electron gun 33 to anelectron-receiving target 34. As an alternative, the ions may be formedexteriorly of the transverse field and propelled into the field as alaminar beam or may be produced in the field in the desired plane byimpingement of a positive ion beam on a solid sample in the mannerdescribed in the aforementioned co-pending application. It is importantto the invention that the ions be formed in a plane of minimum thicknessso that all will be subjected to the same field forces at the moment oforigin and their trajectories will hence originate in a given plane.

The several electrodes 12 through 21 are connected to a voltage divider34 which is, in turn, connected across a voltage source 36 so that theregion defined by the electrodes has a uniform electrical fieldimpressed thereon.

A barrier or resolving electrode 33 is disposed exteriorly of thecrossed fields and is provided with a resolving aperture 38Asubstantially parallel to the so-called boundary aperture defined by theadjacent electrodes 15, 16. A collector electrode 4%] is disposed tocollect ions passing through the resolving aperture 38A and is connectedto a sensing circuit 42, which may be any of the conventional types.

Operation of the instrument of Fig. l is best explained with relation tothe diagram of Fig. 3 in which the dotted line AA represents the planeof .ion formation corresponding to the laminar electron beam 32 ofFig. 1. Three types of cycloi-dal trajectories originating on the planeAA are shown in Fig. l. The boundary aperture defined by the electrodes15, 16 in Fig. l is illustrated simply as a slit S1 .in the diagram ofFig. 3, the resolving aperture in the electrode 38 as the slit S2 givingaccess to collector electrode 40.

In the diagram, ions whose mass M1 is such that their maximumy-coordinate is exactly equal to h will, if formed in the proper regionof the plane AA, emerge from the boundary slit 51 moving in a directionapproximately parallel to AA, where the interrelationship between thevarious parameters is given by where electrostatic units,

These are the ions which will pass through the resolving slit S2 anddischarge at the collector electrode. Particles of lighter mass M2having a trajectory with a maximum ordinate pitch less than 11 cannotemerge from the slit S1 at all as will be seen from the Equation 1.Particles of heavier mass M3 can emerge from the slit S1 only indirections which make an angle with the plane AA greater than themaximum angle made by ions of mass M1. These particles are eliminatedfrom the beam by the resolving slit S2 so that the only ions which canemerge from the crossed fields of the source at such an angle as to passthrough the slit S2 are those of a single well defined mass.

As explained above, this mass discriminating .ion source as constitutedwith resolving and collector electrodes, is unconventional in that theresolution of one mass from another takes place on the basis of theangle at which ions of mixed mass pass from the influence of the crossedfields, i. e. through the boundary aperture, the angle being ditferentfor different masses. Focusing, as the term is usually understood in theart, does not occur and hence the normally objectionable space chargeeffects are minimized.

This so-called directionally sensitive mass spectrometer has animportant advantage over the well known crossed field mass spectrometer.In the conventional instrument there is a region in the neighborhood ofthe resolving slit in which the ions are moving very slowly and henceare susceptible to disturbance by relatively small space charge efiects.In the present instrument, ion movement can be maintained at aconvenient velocity after leaving the boundary slit S1 (Fig. 3) so as tominimize the sensitivity to space charge effects.

The instrument is very sensitive because of the large effective electronbeam area, but is also very sensitive to initial energies. From theexamination of Fig. 3 it is apparent that initial energy of ions formedin the plane AA may disturb their induced relationship to the boundaryaperture S1 so that a lighter ion, the trajectory of which isrepresented by the curve m2 of the figure, may be displaced by suchinitial energies so as to pass through the boundary aperture. By thesame token, a heavier ion, the trajectory of which would normallyprevent it from passing through such an aperture may, as a result ofinitial energies, be caused to pass through the boundary aperture on anaxis approximately parallel to the plane AA. The efiects of initialenergies are particularly noticeable when higher masses are involved andin such circumstances it is desirable to combine the mass discriminatingion source with an associated resolving system. In such a combinationthe ion source not only accomplishes a gross preliminary separation butalso is highly efficient for the reason mentioned above and, whencombined with any of several conventional resolving systems, results ina highly efiicient mass spectrometer of high resolving power. The massdiscriminating feature of this ion source is particularly convenient insuch combinations where high intensities are desired, since the mainresolving system has admitted to it only a limited fraction of the totalion current formed by the ion source. This feature serves to alleviatespace charge effects in the main resolving system.

In Fig. 4 i have shown diagrammatically an ion source in accordance withthe invention combined with a socalled full-wave R. F. gap system. Theion source itself does not dilfer appreciably from the source of Fig. l,the dilference in the apparatus of Fig. 4 lying in the associatedresolving system. Referring to the figure, a source 50 similar to thatof Fig. l is shown including electrodes 51, 52 which define a boundaryaperture S1 as in the foregoing figures. A full-wave R. F. gap system isdefined by the electrodes 54, 55 connected across an oscillator 56. Afull-wave R. P. gap system functions to affect the energy of ionspassing therethrough as a function of specific mass, the energy of allions other than the socalled resonant ions being altered in traversingthe gap system. The so-called resonant ions will traverse the gap systemin exactly one cycle of oscillation and will emerge therefrom withexactly the same energy at which they were introduced. The principle ofoperation of a fullwave gap system is explained at some length in mycopending application Serial No. 275,567, filed March 8, 1952. Asdescribed in this co-pending application a fullwave gap system ishelpless to effectuate separation between ions of difiering mass it theyenter the system at the same velocity. Since the ions issuing from theboundary apertures S1 and the preliminary resolving slit 58A may do soat essentially a uniform velocity, it is necessary to develop anaccelerating field between this boundary aperture and the full-wave gapsystem to remove such velocity degeneracy. This is accomplished byapplication of a high negativebias to the first grid 54 of the fullwavegap system, the accelerating field thus established between electrode 58and grid 54 overcoming any velocity degeneracy of the ions issued fromthe ion source.

In employing the ion source with a full-wave R. F. gap system, as shownin Fig. 4, it is necessary tomodulate the ion beam to provide properphasing and time duration of the ion pulses introduced to the gapsystem. Conveniently such modulation is accomplished by modulation ofthe ionizing electron beam by conventional and familiar means, suchmodulation being synchronized with the periods of alternation of the R.F. gap system itself.

As stated above, ions of non-resonant mass change energy in passingthrough the gap system. A bucking grid 60, provided with a smallpositive bias, will reject all but the most energetic ions emerging fromthe gap system. This means that the so-called resonant ions will berejected as well as less energetic ions, unless the resonant ions are,due to the characteristics of the sample and the band passcharacteristics of the source, the lightest or the heaviest of the ionsinjected into the gap system. The most energetic ions will be either thelightest or the heaviest entering the gap system depending on thephasing of the system. A collector electrode 62 is disposed to collections traversing the bucking grid 60 and is connected to a sensingcircuit 64 which may be of any conventional type. In this system, and asexplained above, the most energetic ions issuing from the source arecollected and sensed, and these may or may not be the resonant ions.

The mass spectrometer of Fig. differs from that of Fig. 4 in beingsensitive to resonant ions regardless of the band pass, characteristicsof the source. In this embodiment, an electrostatic energy filter 68replaces the bucking grid 60 of Fig. 4 and a resolving electrode 70 isinterposed between the velocity filter and collector electrode. In allother respects the devices of the two figures are identical and commonreference numerals are employed. Since all ions other than resonantchanges energy in the full-wave gap system, the energy filter 68 can beoperated to focus the resonant ions through an aperture 70A in theresolving electrode 70 on the collector electrode 62.

The apparatus of Figs 4 and 5 merely illustrate the possibility ofcombining the mass discriminating ion source of the invention with anauxiliary resolving system. There is no inherent limitation tocombination with a full-wave R. F. gap system, it being also feasible toeffectuate further resolution by means of a conventional 90 or 180magnetic analyzer or by the other mass revolving means.

1 claim:

1. An ion source for a mass spectrometer comprising an ionizing chamberhaving an ion outlet aperture, means for establishing a uniformelectrical field across the chamber, means for establishing a magneticfield across the chamber normal to the electric field, means forionizing a sample to be analyzed in a plane spaced from the aperture andnormal to the electric field whereby ions are propelled from the planeof origin under the influence of the electrical and magnetic fields incycloidal trajectories so that certain ions will be propelled throughthe aperture after approximately one-quarter cycle of cycloidal travel.

2. An ion source for a mass spectrometer comprising an ionizing chamberhaving an ion outlet aperture, means for establishing a uniformelectrical field across the chamber, means for establishing a magneticfield across the chamber normal to the electric field, means forionizing a sample to be analyzed, means for orienting the ions in aplane spaced from the aperture and normal to the electric field wherebyions are propelled from the plane under the influence of the electricaland magnetic fields 6 in cycloidal trajectories so that certain ionswill be ro pelled through the aperture after approximately one-quartercycle of cycloidal travel.

3. An ion source for a mass spectrometer comprising an ionizing chamberhaving an ion outlet aperture, means for establishing a uniformelectrical field across the chamber, means for establishing a magneticfield across the chamber normal to the electric field, means forintroducing a sample to be ionized into the chamber, means for directinga laminar electron beam across the chamber in a plane spaced from theaperture and normal to the electric field whereby ions are formed insaid plane and are propelled therefrom under the influence of theelectrical and magnetic fields in cycloidal trajectories so that certainions will be propelled through the aperture after approximatelyone-quarter cycle of cycloidal travel.

4. A mass spectrometer comprising means for establishing a uniformelectric field across a space, means for establishing a magnetic fieldacross the space normal to the electrical field, means forming an outletaperture adjacent a boundary of said space, means for ionizing a sampleto be analyzed in a plane normal to the electrical field and spaced fromthe outlet aperture, ion mass resolving means disposed exteriorly ofsaid space and in the line of travel of at least part of the ionsissuing from the aperture, and ion collection means disposed to collections traversing the resolving means.

5. A mass spectrometer comprising means for establishing a uniformelectric field across a space, means for establishing a magnetic fieldacross the space normal to the electrical field, means forming an outletaperture adjacent a boundary of said space, means for ionizing a sampleto be analyzed, means for orienting the ions in a plane normal to theelectrical field and spaced from the outlet aperture, from which planethey are propelled under the influence of the electrical and magneticfields in cycloidal trajectories so that certain of the ions will reachthe outlet aperture, ion mass resolving means disposed exteriorly ofsaid space and in the line of travel of at least part of the ionsissuing from the aperture, and ion collection means disposed to collections traversing the resolving means.

6. A mass spectrometer according to claim 5 wherein the resolving meanscomprises an apertured electrode positioned with the aperture thereinbeing aligned with said outlet aperture so that only those ionstraveling through the outlet aperture in a given direction with respectto the plane of ion orientation will pass through the aperturedelectrode.

7. A mass spectrometer comprising means for establishing a uniformelectric field across a space, means for establishing a magnetic fieldacross the space normal to the electric field, means for ionizing asample to be analyzed in a plane normal to the electric field wherebyions are propelled from said plane under the influence of the electricaland magnetic field in cycloidal trajectories, means forming an outletaperture adjacent a boundary of said space and spaced from said plane,an apertured electrode disposed outside said space and adjacent theoutlet aperture, and a collector electrode disposed adjacent theapertured electrode and on the side thereof opposite the outletelectrode.

8. A mass spectrometer comprising means for establishing a uniformelectric field across a space, means for establishing a magnetic fieldacross the space normal to the electric field, means for ionizing asample to be analyzed in a plane normal to the electric field wherebyions are propelled from said plane under the influence of the electricaland magnetic field in cycloidal trajectories, means forming an outletaperture adjacent a boundary of said space and spaced from said plane, afull-wave R. F gap system disposed outside said space and adjacent theoutlet aperture, and an electrostatic energy filter, apertured resolvingelectrode and collector electrode arranged serially following the gapsystem.

9. A mass spectrometer comprising means for establishing a uniformelectric field across a space, means for establishing a magnetic fieldacross the space normal to the electric field, means for ionizing asample to be analyzed in a plane normal to the electric field wherebyions are propelled from said plane under the influence of the electricaland magnetic field in cycloidal trajectories, means forming an outletaperture adjacent a boundary of said space and spaced from said plane, afull-Wave R. F. gap system disposed outside said space and adjacent theoutlet aperture, an accelerating gap disposed in the region ReferencesCited in the file of this patent UNITED STATES PATENTS 2,221,467Bleakney Nov. 12, 1940 2,457,162 Langmuir Dec. 28, 1948 2,471,935Coggeshall et al May 31, 1949

1. AN ION SOURCE FOR A MASS SPECTROMETER COMPRISING AN IONIZING CHAMBERHAVING AN ION OUTLET APERTURE, MEANS FOR ESTABLISHING A UNIFORMELECTICAL FIELD ACROSS THE CHAMBER, MEANS FOR ESTABLISHING A MAGNETICFIELD ACROSS THE CHAMBER NORMAL TO THE ELECTRIC FIELD, MEANS FORIONIZING A SAMPLE TO BE ANALYSED IN A PLANE SPACED FROM THE APERTURE ANDNORMAL TO THE ELECTRIC FIELD WHEREBY IONS ARE